This invention relates to an output distortion correction amplifier system.
Voltage feedback amplifier systems typically employ a differential transconductance input stage including a current mirror to drive an output stage which drives a load. The output of the output stage is fed back to the inverting input of the input stage. A compensation capacitor is connected to the high impedance node at the input to the output stage. These amplifier systems work well, drawing little current at their inverting and non-inverting inputs yet supplying at the output stage significant current to drive a load. One problem with these amplifier options is that the output stage is non-linear and introduces distortion due to transistor xcex94Vbe. To combat this problem, in one approach, an internal distortion correction network is introduced between the output of the output stage and the current mirror and input stage, U.S. Pat. No. 5,166,637. This approach adds a buffer stage between the high impedance node and the current mirror, a capacitor between the current mirror and the output of the output stage and also adds a current source in series with the current mirror. This added current source supplies a fixed amount of current. Thus, in systems where the signal can vary widely requiring a wide range of current, this current source can limit the operation and application of the system and limit the voltage headroom. Another issue with that approach is that the error voltage is sensed not directly across the output stage but indirectly using another buffer amplifier and current source.
It is therefore an object of this invention to provide an improved output distortion correction amplifier system.
It is a further object of this invention to provide such an improved output distortion correction amplifier system compatible with voltage feedback amplifiers, or any others that have an output buffer that may introduce xcex94vbe distortion.
It is a further object of this invention to provide such an improved output distortion correction amplifier system which is simple and effective, and does not require adding a current source in series with the current mirror where it would limit the current range and voltage headroom.
It is a further object of this invention to provide such an improved output distortion correction amplifier system which corrects for the distortion locally at the output stage where it occurs and does not interfere with the input stage.
It is a further object of this invention to provide such an improved output distortion correction amplifier system which directly senses the distortion voltage across the output stage.
This invention results from the realization that a simple, more effective distortion correction amplifier system for any amplifier that may experience xcex94Vbe distortion in the output stage can be achieved without added current sources in series with the current mirror and without limiting voltage headroom by sensing the distortion voltage locally, directly across the output stage, generating a current representative of that distortion voltage and providing it to a current mirror associated with the input stage for delivering to the compensation impedance a correction current to develop a correction voltage at the input of the output stage to locally nullify the effect of the distortion voltage without interfering with the input stage.
This invention features an output distortion correction amplifier system including an input stage, a current mirror connected to the input stage, and an output stage having its input connected to the input stage and current mirror and its output connected to the input stage. A compensation impedance is connected to the input of the output stage. A distortion correction circuit directly senses a distortion voltage across the output stage and provides to the current mirror a current representative of the distortion voltage for delivering to the compensation impedance a correction current to develop a correction voltage at the input of the output stage to nullify the effect of the distortion voltage.
In a preferred embodiment, the compensation impedance may include a capacitor. The distortion correction circuit may include at least one buffer amplifier and a nullifying impedance. The nullifying impedance may include a capacitor. The nullifying impedance may be proportional to the compensation impedance and the ratio of the current representative of the distortion voltage to the correction current. The distortion correction circuit may include two-buffer amplifiers with their low impedance outputs connected to the nullifying impedance. The two buffer amplifiers may have their inputs connected across the output stage, and their high impedance outputs connected to the current mirror.